Chelsea are confident of completing the transfer of David Luiz from Benfica within the next 48 hours, with the fee of around £21.5m their biggest outlay on a player since Andriy Shevchenko's £30m arrival from Milan five years ago.

The centre-half's signing is part of an attempt by Carlo Ancelotti to salvage Chelsea's season after the manager admitted before tonight's game at Bolton Wanderers that his team have failed to marry a positive performance and result for almost four months. The champions have not won away in seven matches but Ancelotti goes back even further, to the game at home to Arsenal on 3 October, to pinpoint a good display that yielded victory.

That win had moved Chelsea clear at the top of the Premier League but their worst run of form in 15 years has left them trailing the leaders, Manchester United, by 10 points and effectively forced them into the transfer market. Talks will resume today with Benfica's president, Luís Filipe Vieira, aimed at securing Luiz, with the possibility remaining that a young player could move in the opposite direction, most likely on loan, as part of the deal.

The defenders Jeffrey Bruma or Patrick van Aanholt have been mentioned and Chelsea could explore involving as a makeweight the Serbia defender Slobodan Rajkovic, who is on loan at Vitesse Arnhem but does not qualify for a work permit in England. An agreement with Benfica is edging closer after Vieira spent the weekend in London, along with Luiz's agent, Giuliano Bertolucci.

Luiz was omitted from the Benfica squad for the 4-2 victory over Nacional on Saturday, a clear indication that his club are resigned to losing his services. Chelsea had drawn up a list of alternative targets, believed to include Wolfsburg's Denmark international Simon Kjaer, but are confident they will be able to announce Luiz's eye-catching purchase this week.

Ancelotti will be anxious that the 23-year-old Brazil international, who will be cup-tied in the Champions League, is joining a club who can still realistically retain the Premier League title. Given that the Italian envisages 80 points will be needed to secure the championship – which would mean Chelsea could drop only six more from their final 16 games – a win tonight is imperative.

Asked to identify the last time he had been satisfied with his team's display and the result, Ancelotti said: "Against Arsenal at home, when we played well. I was happy with the way we played at Birmingham last month but not with the result [a 1-0 defeat]. Three and a half months is too long. Too long."

Chelsea, who will be without Frank Lampard with the midfielder's minor calf complaint not to be risked, can take some solace in the fact that they are still to play United twice, as well as Manchester City and Tottenham Hotspur at home.

"We play them and Liverpool at home, and those five games will be key," Ancelotti said. "We have to win them. But the fact that there are five teams going for the title is good for those of us chasing. There is more competition, and therefore more possibility for the teams at the top to drop points. We will try everything to do win the three competitions we are still in."

Ancelotti, who has given the go-ahead for the talks to continue with Fulham over Gaël Kakuta moving there on loan for the rest of the season, admitted he had "made some mistakes" that had contributed to the side's dip in form over the last few months, though he believes the points deficit from United can be clawed back if his team achieve consistency over the second half of the season. He has experience of chasing down a leader as Roma's captain in 1986, albeit that campaign had a sting in its tail.

"I remember that year we needed to close a gap of eight points, with two points for a win, to Juventus over the last two months of the season and we achieved it," said Ancelotti.

"We arrived at the last game level with Juve and had to play Lecce, the team at the bottom, at home. The Lecce manager, Eugenio Fascetti, said before the match that he hated Juventus and was a Roma fan, and there was almost a celebration before the match as if everything would be OK. We ended up losing 3-2 and Juventus went on to win the title. Everything went dark. Such is life."